Finely divided oxide powders are useful in the manufacture of coating compositions, intricately shaped and fine-grained ceramics, cermets, and the like. Small particles are particularly important in the preparation of powder mixtures. In general, the smaller the particle size, the more uniform are the compositions and the better the mechanical properties of metal, ceramic, and cermet articles prepared from the powder mixtures.
Of particular concern for purposes of the present invention are processes for the production of finely divided magnetic particles, i.e., particulate materials that an applied magnetic field can induce to change from a nonmagnetized condition (exhibiting no external fields) into a magnetized condition (exhibiting external fields), and which after removal of the applied magnetic field remain at least partially magnetized in the sense of continuing to exhibit external fields.
As described in U.S. Pat. No. 3,425,666, conventional ferrimagnetic material production involves preparation of polycrystalline magnetic materials in two main steps: (a) preparation of a mixture, as uniform as possible, of the nonferrimagnetic starting materials, and (b) conversion of said starting materials at an elevated temperature to produce the desired ferrimagnetic material by solid state reaction. An example is the solid state reaction of NiO with Fe.sub.2 O.sub.3 at an elevated temperature, to produce the nickel ferrite, NiFe.sub.2 O.sub.4.
In this type of solid state reaction the starting materials generally are prepared in powdered form, placed together, and heated. The heating causes a mutual diffusion of constituents of each starting material and the growth of a crystallite of the desired ferrimagnetic spinel. When the resulting material is needed commercially in solid form, usually the material is powdered again. Thereafter, if a solid shape is desired, the powder is formed into the selected shape and sintered.
Generally the starting materials in the oxide form are mixed together in the desired proportions by dry or wet ball milling. After the milling the material is heated to 500.degree.-800.degree. C., and the resulting material is crushed and milled again. This process can be further repeated to obtain additional homogeneity.
Another procedure involves the decomposition method, in which the starting materials are mixed by milling in the salt form instead of the oxide form, and then the salts are converted to the oxides by thermal decomposition in air.
Another procedure involves the precipitation method, which has been utilized in an attempt to avoid the lengthy milling process of the oxide and decomposition methods. The objective is to precipitate from a solution the required materials simultaneously in either a hydroxide or oxalate form to yield a precipitate containing the required metal hydroxides or metal oxalates in the correct proportions intimately mixed.
The above described oxide, decomposition, and precipitation methods involve various disadvantages. In the oxide and decomposition methods the lengthy ball milling that is required is a disadvantage. Even with extended ball milling there is room for much improvement in the homogeneity of the resulting mixture.
The precipitation methods directionally improve mixture homogeneity, but entail other disadvantages. For example, when a strong base such as sodium hydroxide is used to cause precipitation, the cation must be removed from the resulting mixture to purify it, and this can present a difficult purification problem.
U.S. Pat. No. 3,822,210 describes a process for producing fine spinel-type ferrite particles which are highly dispersible. Spinel-type single-crystal ferrite particles are provided of substantially isotropic shape containing iron and at least one kind of divalent metal other than iron, the ratio of the total number of iron atoms to the divalent metal atoms being at least 2 to 1 and the average particle size ranging from about 0.05 to 1.0 micron. The ferrite crystals are made by admixing an aqueous solution containing ferrous ions and the divalent metal ions with 0.55 to 3 mol equivalents, relative to acid in the solution, of an alkali to obtain a suspension of the hydroxides at a pH of more than 6.5 and thereafter bubbling an oxidizing gas into the suspension maintained at 60.degree. C. to 90.degree. C. until the hydroxides disappear and ferrite particles are formed.
U.S. Pat. No. 4,097,392 describes a manufacturing process for ferrimagnetic materials and pressure-compacted soft ferrite components utilizing a wet process for compositional preparation of materials in which metal carbonates and metal hydroxides are coprecipitated in controllably selected ratios. An aqueous solution of metal ions is formed by dissolving pure metals in acid. This aqueous metal ion solution is added to a predetermined solution of carbonate ions and hydroxide ions. Concentrations, temperature, and rates of addition are controlled to select the ratio of carbonate groups to hydroxide groups in the coprecipitated particles and the size of such particles. The controllably selected ratio of carbonate groups to hydroxide groups facilitates separation of the coprecipitation particles and maintains residual hydroxide groups in the material so as to extend solid-state reactivity of the coprecipitated particles for grain growth and densification purposes until the final heat treatment in which the pressure compacted articles are sintered.
In Bull. Amer. Ceram. Soc., 61(3), 362 (1982) and in Ferrites, Proc., ICF, 3rd [48TRAI] 1980 (Pub. 1982), 23-26, a process is described for the preparation of high performance ferrites from metal acetylacetonates. A solution of iron, zinc, and manganese acetylacetonates in ethanol is refluxed for one hour. The solution is treated with ammonium hydroxide to a pH level of 10-11, and the treated solution is refluxed two hours to precipitate solids. The solids are recovered, microwave dried, calcined for five hours at 500.degree. C. under nitrogen, and then shaped and fired for another hour under nitrogen.
Journal of the Society of Powder and Powder Metallurgy, 29(5), 170 (1982) describes the preparation of crystalline BaFe.sub.2 O.sub.4 and BaFe.sub.12 O.sub.19 ferrites via an amorphous state by calcination of gels which are obtained by hydrolysis of a metal alkoxide mixture.
Other literature of interest relates to the production of conducting oxide powders by an "amorphous citrate" type process.
Science of Ceramics, Vol. 8, British Ceramic Society, Stoke-on-Trent, England (1976) describes the preparation of ferrite powders (e.g., LiMnZnFe.sub.2 O.sub.4) suitable for memory cores, involving the pyrolysis of amorphous organometallic precursors obtained by evaporation of metallic salt solutions.
In Powder Metallurgy, 22, 14 (1979), conducting oxide powders are prepared by dehydration of citrate-nitrate gels at 70.degree. C. to provide solid precursor materials, which are then pyrolyzed to yield a semiconducting composition such as lanthanum chromite doped with strontium. A similar procedure is employed to synthesize strontium-substituted lanthanum manganite perovskite powder as reported in Journal of Materials Science, 17, 2757 (1982).
There remains a need for new and improved processes for the production of fine grain inorganic oxide powders with electrical or magnetic properties such as ferrimagnetic spinel compositions.
Accordingly, it is an object of this invention to provide an improved procedure via an organometallic intermediate for the production of a ferrimagnetic spinel composition having an average particle size of less than about 1000 angstroms.
It is another object of this invention to provide a ferrimagnetic spinel composition having a ferrite crystal lattice structure of improved dimensional stability and strength, and which exhibits improved magnetic properties such as permeability and loss factor.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.